Extension Tree Fruit Team

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Early Season Disease Development Situation

Posted: April 23, 2012

Apple scab lesions appeared April 17 on trees that were not adequately protected March 20-25. This could have led to severe secondary infection April 18-23. Proper and thorough coverage at this time of the year is critical for preventing infection of young fruits. This article contains updates on pome and stone fruit diseases and new fungicide properties.

Apple Scab Infection Periods through April 23. Infection period models for other diseases may be viewed at http://extension.psu.edu/fruit-times.

Dr. Keith S. Yoder, Virginia Tech AREC Plant Pathologist

Apple Scab

Scab ascospores were first trapped with rains March 19, but it was not an infection period. Early scab infection days (as indicated on the Penn State FREC web site) occurred Mar 20-25. Spore discharge was extremely high Mar 22. Hopefully copper spray covered several of these. Another primary infection period occurred Mar 31-Apr 1. SkyBit indicates that most of the ascospores have matured by now and were likely discharged by recent rains. But typically, a few scab lesions in the tree could produce more secondary spores than the amount of primary inoculum we would expect to overwinter on the ground.

Rusts

If you are in an area prone to cedar-apple or quince rusts, many of the above wetting periods, Mar 20-25 and Apr 1, Apr 15 and 18-21, were also favorable for rust infection. In potentially unprotected rust situations, it is important to follow likely rust infection periods with an SI fungicide for after-infection control of rusts.

Powdery Mildew

Wetting periods call our attention to scab infection, but mildew infection sneaks by almost unnoticed on dry days above 53F. Mildew spores were likely available on infected emerging buds by Mar 23. Since that date there may have been as many as 17 dry weather “mildew infection days” (Mar 25-27and 29 and Apr 3-8, 12-14, 16-17, and 19-20). Last week we saw secondary mildew lesions in the Winchester area, evidence of this disease’s rampant early season activity.

Fire Blight Outlook

There was indicated severe risk fire blight blossom infection at Biglerville Mar 29 and Apr 15-17. Continue to follow updates and apply streptomycin to late bloom before likely infection events.

Stone Fruit

After green tissue is first exposed, typically an apple scab infection period will also be a cherry leaf spot period. Likely infection periods occurred Mar 20 and 25, and Apr 20-22, and more infection periods are predicted for this week. Unlike apple scab, leaf spot infection occurs through stomates and all leaves are susceptible later in the season. Bacterial spot is favored by warm temperatures (70-85 F) with light rains or heavy dew (like fire blight). Windy weather is most conducive for disease development and spread and infections occur only when the leaves are wet. Now is the time to control infection by peach scab, which requires about six weeks for symptoms to show.

Fungicide Registration Updates

Some fungicides that you probably heard a lot about during the winter, Luna Sensation, Luna Tranquility, and Fontelis are now fully registered. Here are some considerations as related to our perspective from research we conducted in Winchester, Virginia.

Luna Sensation is a package mix of fluopyram (SDHI, Group7) and trifloxystrobin (Group 11). Luna Sensation has excellent broad spectrum control. It has been the best treatment in our mildew tests at Winchester. Its most significant weakness is on quince rust. Because one of its components is a strobilurin (QoI, Group 11) fungicide like Flint and Sovran, resistance issues in apple scab and powdery mildew could impact its effectiveness compared to where resistance is not a problem. Luna Sensation is now registered on apples at 4-5.8 fl oz/A. Restrictions include a limit of 4 applications, 21 fl oz/A/year; 14-day PHI, and a 12-hour REI. I suggest using the 4 fl oz/A rate and combining Luna Sensation with a protectant such as an EBDC to back it up for rusts and QoI-resistant scab. Where rusts are a serious problem, consider including an SI fungicide (e.g., Rally, Topguard, Indar, Inspire Super) as supplemental tank-mixes or alternating schedules with Luna Sensation.

Luna Tranquility is a package mix of fluopyram (SDHI, Group7) and pyrimethanil (Group 9). Luna Tranquility is now registered on apples at 11.2-16 fl oz/A. Restrictions include a limit of 54.7 fl oz/A/year, a 72-day pre-harvest interval and a 12-hour REI. Because this mix has been effective on QoI-resistant scab in Michigan, we suggest combining Luna Tranquility with a protectant (e.g. EBDC) and trying this as part of the scab control program in locations where there is suspected QoI (strobilurin) resistance in apple scab. Be aware that Group 9 (AP, Scala / Vangard) resistance may be an unknown factor in an area. We have not tested this combination product previously, but are now testing the 11.2 fl oz/A rate. We expect that Luna Tranquility will be weaker than Luna Sensation on mildew and will also have a similar weakness on rust control.

Fontelis (penthiopyrad, SDHI, Group 7) is now registered at 10-12 fl oz/A for scab, in combination with an EBDC and other materials at 14-20 fl oz/A for mildew, rusts and Alternaria leaf spots. Fontelis has a limit of 61 fl oz/A/year; a 28-day PHI and a 12-hour REI. Because Fontelis is a single compound, it may provide more flexibility for scheduling. Although Fontelis is registered at 10-20 fl oz/A, in our tests we have found a drop off in effectiveness at the lower rates and suggest a minimum rate of 14 fl oz /A. We view Fontelis primarily as a mixing partner with an EBDC or QoI or other materials for scab. Like both Luna Sensation and Luna Tranquility, Fontelis will be weaker on rusts than the SI fungicides. We have found that Fontelis activity on mildew is improved by combining it with oil.

As with many newer fungicides, note that all of the above SDHI products (Luna Sensation, Luna Tranquility, and Fontelis) suggest or require no more than two sequential applications before rotating to a different chemical class.

An old apple scab fungicide, Syllit (dodine), previously had a restriction of not feeding Syllit-treated pomace to livestock. This restriction has now been removed from the label.

Where trade names appear, no discrimination is intended, and no endorsement by Penn State Cooperative Extension is implied. Recommendations based on conditions observed at Penn State Fruit Research and Extension Center, Biglerville, PA.